A viable hybrid between two species of the toad genus Xenopus will be used to study the activity of the nucleolus in early development. Observations are to be made on the variability of nucleolar expression as a function of time and tissue-type, and analysis of the variability will be performed using genetic and molecular hybridization techniques. Embryonic sex cell transfers will be used to test the autonomy of reproductive cells and to determine the source and time of synthesis of certain enzymes, using a methodology based on the existence of isozymes. Hybrid animals, as well as the practice of nuclear transplantation with overripe oocytes, will also be used in a study of the sources of sterility.